mapM と State モナドで mapAccumL

mapAccumL.hs

#!/usr/bin/env stack

import Control.Monad.State
import Data.List
import Data.Tuple

myMapAccumL :: (Int -> Int -> (Int, Int)) -> Int -> [Int] -> (Int, [Int])
myMapAccumL f s0 xs =
  swap
    $ runState
      ( mapM
          ( \x -> do
              acc <- get
              let (acc', x') = f acc x
              put acc'
              return x'
          )
          xs
      )
      s0

-- ([1, 2, 3], 6)
main :: IO ()
main = do
  -- (6, [1,2,3])
  print $ mapAccumL (\acc x -> (acc + x, x)) (0 :: Int) [1, 2, 3]

  -- (6, [1,2,3])
  print $ myMapAccumL (\acc x -> (acc + x, x)) (0 :: Int) [1, 2, 3]

短縮版ができた！！！！！

#!/usr/bin/env stack

import Control.Monad.State
import Data.List

main :: IO ()
main = do
  -- ([1,2,3], 6)
  print $ runState (mapM (\x -> state $ \acc -> (x, x + acc)) [1, 2, 3]) (0 :: Int)

  -- ([1,3,6], 6): 数列 → 累積和
  print $ runState (mapM (\x -> state $ \acc -> (x + acc, x + acc)) [1, 2, 3]) (0 :: Int)

  -- ([1,2,3], 6): 累積和 → 数列
  print $ runState (mapM (\x -> state $ \last -> (x - last, x)) [1, 3, 6]) (0 :: Int)